Protozoan parasites cause some of the most devastating diseases world-wide. The parasites responsible for infectious diseases in man and animals, including fish, include those of the genera Giardia, Trichomonas, Leishmania, Trypanosoma, Crithidia, Herpetomonas, Leptomonas, Histomonas, Eimeria, Isopora, Neospora and Plasmodium. 
Plasmodium falciparum is the organism that causes malaria in humans, and continues to be responsible for more than one million deaths per year. Drug resistance is increasing even to newer antimalarials such as mefloquine. This has led to an urgent need for new antimalarials both for chemotherapy and prophylaxis.
One feature common to protozoan parasites is that they do not have the ability to synthesise purines de novo and rely upon purine salvage and purine recycling to meet their purine needs. Purines are essential for the survival and replication of protozoan parasites, so they must obtain them from their mammalian hosts, which are able to synthesise purines by de novo pathways. The disruption of purine salvage pathways is therefore considered to be a means to specifically target protozoan parasite infections. Malaria is of particular interest, in that it causes the greatest economic and social harm.
Prior studies have shown that inhibitors of purine salvage enzymes kill malaria. Blocking P. falciparum purine nucleoside phosphorylase (PfPNP) with Immucillin-H, a transition state inhibitor based on inosine, induces starvation of purine leading to death of the organism. Because the transition state structures of both human and P. falciparum PNP enzymes are similar, transition state analogues based on inosine, such as Immucillin-H, inhibit both the human and P. falciparum PNPs.
P. falciparum is remarkable because of its small number of purine salvage enzymes despite the complete reliance on this pathway. The inventors have recently reported that the purine salvage enzymes P. falciparum adenosine deaminase (PfADA) and PfPNP each have two roles in the parasite and replace the functions in mammals of purine nucleoside phosphorylase (PNP), adenosine deaminase (ADA), methylthioadenosine phosphorylase (MTAP), adenosine phosphoribosyltransferase (APRT) and adenosine kinase (AK). The actions of PfADA and PfPNP permit the parasite to form hypoxanthine from erythrocyte purine pools and to recycle hypoxanthine from polyamine synthesis within the parasite. Hypoxanthine is a precursor for all purines and is a central metabolite for nucleic acid synthesis in P. falciparum. 
5′-Methylthioadenosine is formed as a product of polyamine synthesis. In P. falciparum, it is recycled in a pathway in which PfADA converts it to 5′-methylthioinosine, then PfPNP converts 5′-methylthioinosine (and inosine) to hypoxanthine.
Following the observation that PfPNP uses 5′-methylthioinosine as a substrate, but human PNP (HsPNP) does not, the inventors recently synthesized transition state analogue inhibitors based on 5′-methylthioinosine (in particular 5′-methylthio-Immucillin-H) and reported them to be the first potent inhibitors that are selective for PfPNP relative to HsPNP. 5′-Methylthio-Immucillin-H showed 112-fold specificity for PfPNP. Further, 5′-methylthio-Immucillin-H was shown to kill P. falciparum in culture (see J. Biol. Chem., 2005, 278, 9547-9554 and references therein).
The inhibition of PfADA has also been investigated. Coformycin, 2′-deoxycoformycin and the L-ribosyl analogues of the coformycins are known to be tight-binding inhibitors of both mammalian and P. falciparum ADAs [Daddona, P. E., Wiesmann, W. P., Lambros, C., Kelley, W. N., and Webster, H. K. (1984) J Biol Chem 259, 1472-1475; Wilson, D. K., Rudolph, F. B., and Quiocho, F. A (1991) Science 252, 1278-1284]. Coformycin and 2′-deoxycoformycin have comparable activity against bovine ADA and PfADA (J. Biol. Chem., 2005, 278, 9547-9554). A single dose of 2′-deoxycoformycin dramatically reduced parasitemia in primates with Plasmodium knowlesi [Webster, H. K., Wiesmann, W. P., and Pavia, C. S. (1984) Adv. Exp. Med. Biol., 165 Pt A, 225-229], but 2′-deoxycoformycin is highly toxic in mammals. The challenge then was to discover a potent selective inhibitor of PfADA.

Coformycin can be prepared by various methods. See for example: Thomas, H. Jeanette; Riordan, James M.; Montgomery, John A, Nucleosides & Nucleotides 1986, 5(4), 431-9; Hawkins, L. D.; Hanvey, J. C.; Boyd, F. L., Jr.; Baker, David C.; Showalter, H. D. Hollis, Nucleosides Nucleotides 1983, 2(5), 479-94; Ohno, Masaji; Yagisawa, Naomasa; Shibahara, Seiji; Kondo, Shinichi; Maeda, Kenji; Umezawa, Hamao, J. Am. Chem. Soc. 1974, 96(13), 4326-7; Shimazaki, Masami; Kondo, Shinichi; Maeda, Kenji; Ohno, Masaji; Umezawa, Hamao, J. Antibiotics, 1979 32, 537-538; Yamazaki, Masakuni; Harada, Takashi; Shibuya, Kyoichi; Hayashi, Emiko; Saito, Seiichi; Shimada, Nobuyoshi, Jpn. Kokai Tokkyo Koho (1988), JP 63226296 A2 19880920 CAN 110:73872; Fr. Demande (1978), FR 2383966 19781013 CAN 91:57420; Umezawa, Hamao; Maeda, Kenji; Kondo, Shinichi. Ger. Offen. (1975), DE 2453649 CAN 83:59226; Umezawa, Hamao; Niida, Taro; Niwa, Tomizo; Tsuruoka, Takashi; Ezaki, Norio; Shomura, Takashi. Jpn. Tokkyo Koho (1970), JP 45012278 CAN 73:65025.
It has now been surprisingly found that certain analogues of coformycin are active against PfADA and are therefore potential therapeutic agents for the treatment or prevention of protozoan parasite infections including malaria.
It is therefore an object of the present invention to provide novel coformycin analogues for use against protozoan parasite-infections, especially malaria, or to at least provide a useful choice.
Statements of Invention
In a first aspect the invention provides a compound of formula (I):
where                R1 is selected from an alkyl, aralkyl and aryl group each of which may be optionally substituted by one or more halogen atoms or one or more hydroxyl, amino, or carboxylic acid groups;        X is selected from hydrogen, hydroxyl and halogen;        Y is selected from hydrogen and hydroxyl;        Z is an oxygen atom or a methylene group;        B is the radical of formula (II):        
or a pharmaceutically acceptable salt thereof, or a prodrug thereof.
It is preferred that the compound of formula (I) is a compound of formula (IA):
where R1, X, Y, Z, and B are as defined above.
Alternatively, it is preferred that the compound of formula (I) is a compound of formula (IB):
where R1, X, Y, Z, and B are as defined above.
Preferably R1 is an alkyl group. In a preferred embodiment, R1 is methyl.
X and Y are both preferably hydroxyl. Alternatively, X may be hydroxyl and Y may be hydrogen, or X may be hydrogen and Y may be hydroxyl.
Z is preferably an oxygen atom
Preferred compounds of the invention are:    (i) 5′-methylthiocoformycin [(8R)-8-hydroxy-3-(5-methylthio-β-D-ribofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepine];    (ii) 2′-deoxy-5′-methylthiocoformycin [(8R)-8-hydroxy-3-(2-deoxy-5-methylthio-β-D-erythro-pentofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepine];    (iii) 3′-deoxy-5′-methylthiocoformycin [(8R)-8-hydroxy-3-(3-deoxy-5-methylthio-β-D-ribofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepine];    (iv) 2′-deoxy-5′-propylthiocoformycin [[(8R)-8-hydroxy-3-(5-propylthio-β-D-erythro-pentofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepine]; and    (v) 2′-deoxy-5′-phenylthiocoformycin [[(8R)-8-hydroxy-3-(5-phenylthio-β-D-erythro-pentofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepine].
An especially preferred compound of the invention is 5′-methylthiocoformycin [(8R)-8-hydroxy-3-(5-methylthio-β-D-ribofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepine].
In a second aspect of the invention there is provided a pharmaceutical composition comprising a pharmaceutically effective amount of a compound of the formula (I).
Preferably the pharmaceutical composition comprises:    (i) 5′-methylthiocoformycin [(8R)-8-hydroxy-3-(5-methylthio-β-D-ribofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepine];    (ii) 2′-deoxy-5′-methylthiocoformycin [(8R)-8-hydroxy-3-(2-deoxy-5-methylthio-β-D-erythro-pentofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepine];    (iii) 3′-deoxy-5′-methylthiocoformycin [(8R)-8-hydroxy-3-(3-deoxy-5-methylthio-β-D-ribofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepine];    (iv) 2′-deoxy-5′-propylthiocoformycin [[(8R)-8-hydroxy-3-(5-propylthio-β-D-erythro-pentofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepine]; or    (v) 2′-deoxy-5′-phenylthiocoformycin [[(8R)-8-hydroxy-3-(5-phenylthio-β-D-erythro-pentofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepine].
Most preferably the pharmaceutical composition comprises 5′-methylthiocoformycin [(8R)-8-hydroxy-3-(5-methylthio->D-ribofuranosyl)-3,6,7,8-tetrahydroimidazo[4,5-d][1,3]diazepine].
In another aspect of the invention there is provided a method of treating or preventing a protozoan parasite infection, especially malaria, comprising administering a pharmaceutically effective amount of a compound of formula (I) to a patient requiring treatment. The infection may be caused by any protozoan parasite including those of the genera Giardia, Trichomonas, Leishmania, Trypanosoma, Crithidia, Herpetomonas, Leptomonas, Histomonas, Eimeria, Isopora, Neospora, and Plasmodium. In a preferred embodiment the infection is malaria.
In another aspect the invention provides the use of a compound of formula (I) for the manufacture of a medicament for treating a protozoan parasite infection, especially malaria.